Solid end products of harmful water-containing waste materials and cement with or without additives such as metal oxides, metal hydroxides, silicates of the alkaline or alkaline-earth metals or aluminosilicates, have been known for some time (DE-AS No. 2,426,641). Such solid products, however, contain only a relatively small amount of harmful wastes, generally less than 10% by weight based on the dry weight of the waste in the end product or the end product may have a water content of substantially more than 45% by weight on a water-to-cement basis or both.
Harmful water-containing wastes are aqueous solutions or slurries such as slurries having suspended therein compounds which are harmful or dangerous for the biocycle, for example, poisonous or radioactive compounds. The solid end products of this type known so far have certain limits given by either the water-to-cement ratio, that is, the water-to-fixation compound ratio or the charging of the fixation compounds with waste.
The smaller the volume of aqueous waste in relation to the fixation compounds, the smaller is the portion of harmful waste in the end product and also, the smaller is the relative amount of water-to-fixation compounds (weight percent to weight percent) in the end product.
If the volume of the aqueous waste to be solidified is increased while the amount of fixation compounds remains unchanged, then the weight percent of dry waste is increased in the end product but the weight ratio of water-to-fixation compounds is also increased which, however, is only possible to a point that depends on the type of fixation matrix. Any amount of liquid to be solidified beyond that point cannot be contained by the fixation matrix. As a consequence of an addition of an excessive amount of liquid, the fixation matrix will not solidify or, if the fixation matrix can solidify, it will bleed liquid, that is, water or watery solutions will be left over after solidification of the matrix. In this connection, it is not foreseeable whether this bleed liquid contains harmful waste materials. Certainly, it is not reasonable to undergo a solidification process and, after solidification, have harmful liquid waste left over.
The aqueous waste may be concentrated, for example, aqueous salt solutions may be subjected to evaporation so that the concentration of harmful waste in the end product is increased by a certain amount but this may cause problems with regard to the quality of solidification. In order to obtain a sufficiently large concentration of waste material, the aqueous solutions would have to be concentrated to such a degree that mechanical problems would occur which may render the process impossible to practice; for example, crystallization, transport problems, clogging by dried compounds might occur. The salt content of the end products which are made in accordance with presently known methods, particularly by encapsulating in barrels with cement, is limited by the salt concentration in the waste solution. The solidification of aqueous waste with a content of harmful salts of about 40% by weight leads to solid end products with a salt content of not more than 16% by weight in spite of the fact that the waste-to-cement ratio is about 0.4.
The higher the water-cement value, that is, the weight ratio of water-to-solidification compound, in the end product, the smaller is the resistance of the solid end product to leaching by water or salt solutions. In addition to high resistance against leaching, it is necessary that the solid end product has also a relatively high mechanical strength. Of solid end products containing radioactive waste, a compression strength of 10 N/mm.sup.2 or more is required. Also, for harmful materials there should be:
on the average, a leaching rate of better than 10.sup.-3 g/cm.sup.2 per day (when measured over the period of a year), PA1 a differential leaching rate of better than 10.sup.-4 g/cm.sup.2 per day (as measured at the end of the first year), and PA1 a diffusion parameter of better than 10.sup.-4 cm.sup.2 .times.d.sup.-1. PA1 (a) contains 33 to 50% by weight of harmful waste materials based on the dry weight of the waste in the end product (solid fixation products), and PA1 (b) has a water content corresponding to a water-to-cement ratio of 0.20 to 0.40.
Only if these conditions are fulfilled, the end product may be considered leach resistant.
It is the object of the present invention to provide a solid fixation product of harmful aqueous waste materials and a fixation matrix on the basis of an inorganic solidification compound, which fixation product is resistant to leaching by water and water solutions and is capable of containing a relatively large amount of waste in combination with a low water-to-solidification compound ratio. The fixation products shall have good mechanical properties, a good leach resistance and at least the retaining qualities of the known fixation products (with much lower content of harmful waste in the solid end product). The novel solid fixation product shall be producible in a continuous as well as in a batch-type process and, in any case, shall have especially crushing strength and leach resistance. It is, furthermore, an object of the present invention to provide a method of making such solid fixation products.